1. Field of the Invention
The present invention relates to a solid-state imaging device, a method for producing the device, and a camera including the solid-state imaging device.
2. Description of the Related Art
It is known that in solid-state imaging devices, such as charge-coupled device (CCD) image sensors and complementary metal oxide semiconductor (CMOS) image sensors, crystal defects in photodiodes serving as photoreceivers and interface states at interfaces each between the photoreceiver and an insulating film on the photoreceiver function as sources of dark current. A buried photodiode structure is effectively used in order to inhibit the generation of dark current due to the interface states. The buried photodiode includes an n-type semiconductor region and a shallow p-type semiconductor region (hole accumulation region) for inhibiting the dark current, the p-type semiconductor region having a high impurity concentration and being disposed on the surface of the n-type semiconductor region, i.e., disposed in the vicinity of the interface between the n-type semiconductor region and the insulating film. A method for producing the buried photodiode generally includes implanting B ions or BF2 ions serving as p-type impurities; and performing annealing to form a p-type semiconductor region in the vicinity of the interface between the n-type semiconductor region and the insulating film that constitute the photodiode.
However, in the case of the formation of the buried photodiode by a known ion implantation, heat treatment at a temperature as high as 700° C. or higher is essential for the activation of impurities. Thus, in a low-temperature process at a temperature of 400° C. or lower, it is difficult to form the p-type semiconductor region by ion implantation. Furthermore, in view of the inhibition of diffusion of impurities, the method for forming the p-type semiconductor region by ion implantation and annealing with activation at a high temperature for a long period of time is not desirable.
In CMOS image sensors, pixels each include a photodiode and various transistors, such as a read transistor, a reset transistor, and an amplifying transistor. A signal photoelectrically converted with the photodiode is processed with the transistors. Each pixel is overlaid with a wiring layer including multiple metal-lead sublayers. The wiring layer is overlaid with a color filter that specifies the wavelength of light incident on the photodiode and an on-chip lens that converges light on the photodiode.
In CMOS image sensors, the leads on the pixels disadvantageously blocks light, thus reducing sensitivity. When light reflected from the leads is incident on adjacent pixels, color mixture and the like are caused. Thus, Japanese Unexamined Patent Application Publication No. 2003-31785 discloses a backside-illumination solid-state imaging device that photoelectrically converts light incident from the backside of a silicon substrate including photodiodes and various transistors, the silicon substrate having a thickness reduced by polishing the backside thereof. As described above, the photodiode includes the shallow p-type semiconductor region (hole accumulation region) for inhibiting the dark current, the p-type semiconductor region having a high-impurity concentration. In the backside-illumination solid-state imaging device, the hole accumulation region is disposed at each of front and back sides of the substrate.
However, ion implantation limits the formation of the shallow p-type semiconductor region having high-impurity concentration. Thus, a further increase in impurity concentration in the p-type semiconductor region in order to inhibit dark current deepens the p-type semiconductor region. The deep p-type semiconductor region may degrade the read ability of the transfer gate because the pn junction of the photodiode is remote from the transfer gate.